Charges of fiber reinforced plastic resin are used in the formation of a wide variety of articles. These charges may take the form of one or more sheets of sheet molding compound (SMC) material, various compositions of bulk molding compound material, and in some instances the charge may contain resin impregnated fiber strands in combination with plies of SMC material or the like. Various other charge compositions are known in the art. Typically, the charge is compression molded between two heated dies for a sufficient period of time to allow the resin to cure. The dies are then opened and the part removed.
In general, higher quality parts can be molded from charges where the resin has uniformly impregnated the fibers, while keeping the amount of air and other debris which may become entrapped in the charge to a minimum. The following patents comprise a representative, though not exhaustive, list of known techniques for impregnating reinforcing fibers with resin:
U.S. Pat. No. 4,220,496 to Carley et al; PA1 U.S. Pat. No. 4,220,497 to Carley; PA1 U.S. Pat. No. 4,294,490 to Woelfel; PA1 U.S. Pat. No. 4,294,639 to Woelfel et al; and PA1 U.S. Pat. No. 4,376,749 to Woelfel.
These patents disclose the use of an "open cell" bath to impregnate the reinforcing fibers with resin. These open cell bath constructions are generally characterized by a coverless trough holding the resin. The fibers enter the trough and are bent at a substantial angle by one or more rods which attempt to keep the fiber strands submerged in the bath of resin as they are pulled therethrough.
Open cell constructions such as these have several drawbacks. Air has a tendency to become introduced into the resin bath during operation. This air can become or remain entrapped between the fibers of each strand. As noted above, entrapped air within the charge can degrade the quality of the finished part. Another problem with the open cell constructions is that a wave of resin tends to be created toward the exit of the cell as the fibers are drawn through it. Consequently, the fibers entering the cell may not be submerged in the resin and it becomes difficult to control the length of time that the fibers are actually being impregnated with the resin. The resin characteristics are sometimes difficult to control using the prior art cells, as is the removal of frayed fibers or other contaminants. In addition, the rather substantial degree of bending of the fibers in the known cells has a tendency to fray the more fragile fibers such as graphite and glass. The frayed fibers add to the contaminants in the resin bath and can degrade the structural integrity of the fibers in the finished part.
Those skilled in the art are most likely aware of still other problems with the open cell constructions which are common in the industry.